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- [Instructor] Walter Cannon was an American doctor in the early 1900s who spent a lot of his career expanding our understanding of homeostasis, which is the tendency of our body to respond to the environment in a way that keeps the internal workings of our body stable So the body's ability to maintain the right temperature, the body's ability to maintain the right pH, et cetera. So as a part of his study of homeostasis, Dr. Cannon was really interested in the homeostatic response of animals to threats or dangers. So threats or dangers. And threats or dangers are what we've been referring to as stressors. So basically Dr. Cannon was interested in our homeostatic response to stressors. And he coined the response that he saw The Fight or Flight Response. Because he basically saw that we were either going to prepare to fight against these threats or dangers, or we were gonna prepare to run away from these threats or dangers. So the fight or flight response that Cannon was so interested in, it revolves around the inner workings of the nervous system, and the endocrine system. So on the nervous system side of things, we have the sympathetic nervous system. So we have our brain and our spinal cord that make up the central part of our nervous system. And then nerves branch off of this central core. Part of our nervous system that we don't always think about or aren't thinking about to control is the autonomic nervous system. A branch of that autonomic nervous system that's going on behind the scenes is the sympathetic nervous system. And this is the aspect of our nervous system that's gonna transition our body into the get out of danger mode. And so, with that we'll see increased heart rate in order to pump blood with oxygen and nutrients all around our body. So we have an increased heart rate, that heart's just gonna beat a lot faster. And we're also gonna see increased respirations. So we have an increased respiratory rate. And that gives us the ability to get more oxygen into our blood and all the way to our tissues. And it gives us the ability to expel more carbon dioxide as a waste product from our muscle work. And then we're also gonna see increased peripheral vasoconstriction. So the blood vessels in our peripheral parts of our body, like our arms and our legs, are gonna clamp down and they're gonna tighten in order to push more blood to our core area. Because our arms and legs are important, but we can live without them. It would be a lot harder to live without our brain or our vital organs, so we need a lot of our blood forced into those really crucial areas. So we're gonna see peripheral vasoconstriction happening in this fight or flight response. And then lastly, we're gonna turn off less important things, like digestion and immune function and ovulation, because these things are great if we're just hanging out, but they're definitely less crucial in life or death situations. So teaming up with the sympathetic nervous system, we have the endocrine system. And so the endocrine system is gonna supply some of the hormones. Which, remember, are those chemical messages in order to rally the troops around the body. And the two biggest hormone players actually come out of the same organ, which sit on top of our kidneys, the same organs. And those are your adrenal glands. And so the inside of our adrenal glands, this hash part, is called the medulla. And from the medulla we get our catecholamine hormones. And our catecholamine hormones include epinephrine and norepinephrine. So sometimes those are more often called adrenaline and noradrenaline. But these are the hormones that are major communicators for increasing the heart rate and blood pressure, and otherwise communicating that sympathetic response that we just talked about. So those are really supporting the sympathetic nervous system, these hormones coming out of the adrenal medulla. And then on the outside, this bolder part of our adrenal gland, that's called the cortex. And out of the cortex comes some other major stress hormones. And the biggest one is a glucocorticoid called cortisol. And it helps me remember that cortisol comes from the cortex if I'm thinking about that cort, kind of root word, or root part of the word. But cortisol is a steroid hormone which contributes to the stress response by redistributing glucose energy in the body and suppressing the immune system. And so just to review real quick, we have this fight or flight response to stressors. So we're preparing to either fight or to run away. And that's being communicated by our sympathetic nervous system, which is being assisted by the hormones of our endocrine system. And so we have this fight or flight response that Dr. Cannon coined the terminology of. But we also have another response called Tend and Befriend. We're calling this affiliative response the tend or befriend response. Because it turns out that sometimes a better response to stress is to huddle together and to form support systems. And an important biological component to this response is the hormone oxytocin. And oxytocin is associated with things like pair bonding. And so it can moderate this stress response. And it's really interesting if you know much about oxytocin, it's majorly integrated with the hormone estrogen. And so estrogen is a major sex hormone in women. And so what follows is kind of a naturally greater disposition to this tend and befriend response in women much more so than in men. And so we have these two major categories of response to stressors. We have this fight or flight response and this tend and befriend response. And in the later part of the 20th century a guy named Hans Selye continued the stress research of Dr. Cannon. And he was able to classify distinct stages to the adaptive fight or flight and tend and befriend responses. And he called this generalization the General Adaptation Syndrome. So I'm gonna shorten that to G.A.S. For General Adaptation Syndrome. And he saw this General Adaptation Syndrome as having three phases. And in the first phase, he called it the alarm phase. And in this alarm phase, that stress reaction kicks in. So your heart starts racing, and your resources are mobilizing. You're ready to fight or to run for your life. And that's the alarm phase. In the second phase, which he called the resistance phase, we're actually fighting or fleeing, or huddling together. Our temperature is elevated and our blood pressure and rate of breathing remain high. And we're also bathing our body in the stress hormones like cortisol. And so our body has reserves to react to those acute stressors in this resistance phase, but those reserves only go so far. And that brings us to the third phase of the G.A.S., and that's exhaustion. So if the resistance stage isn't followed by recovery, the body's stress resources get depleted and it leads to exhaustion. And our tissue and muscles become damaged and our dampened immunity can make us really susceptible to illness. And all around over exposure to stress can have some really damaging effects. Because like many animals, our body's pretty well equipped to handle short term stress, but the problem is that when we apply those life saving stress reactions to our daily commute and to our 9 to 5 jobs, and to other psychosocial trivialities of life, we end up spending way too much time experiencing the negative consequences of stress, this exhaustion consequence. And it has some pretty nasty consequences. Next up we're gonna explore what happens to our body's behaviors and emotions when we're exposed to chronic stress.